The present invention relates to a method of electrophotographically producing a visual toner image wherein an electrostatic latent image on a rotating photoconductive drum (image-bearing member) is developed by a magnetic developer attractively retained on the surface of a developer transporting means formed from a cylindrical permanent magnet member. In particular, an electrophotographic visual image-forming method by which the adhesion of a carrier in the magnetic developer to the surface of the photoconductive drum can be effectively avoided.
In a known electrophotographic imaging process and electrostatic recording process utilized in printers, facsimile machines, etc., an electrostatic latent image is formed on the surface of a cylindrical photoconductive drum. A developing roll composed of a sleeve and a permanent magnet mounted interiorly of the sleeve and rotatably relative to the sleeve is disposed opposite to the photoconductive drum. A magnetic developer is magnetically attracted on the surface of the sleeve and transported by the relative rotation of the sleeve and the magnet. The magnetic developer transported to a developing zone forms a magnetic brush which brushes the surface of the photoconductive drum to develop the electrostatic latent image to a visual toner image. The toner image is transferred onto a recording sheet which is then heated to permanently fix the toner image thereon.
In the conventional image forming apparatus, a corona discharging method, in which a high voltage such as DC 5-8 kV is applied to a metal wire to generate corona, is employed to charge the photoconductive surface to a uniform potential and transfer the toner image onto the recording sheet. However, the corona discharge is accompanied with undesired by-products such as ozone, nitrogen oxides (NOx), etc. to cause air pollution due to discomfortable odor, etc. The undesired by-products change the properties of the photoconductive surface to reproduce obscure or blurred images. Further, when the corona wire is stained, the toner images with non-developed portion, undesired toner lines on the background, etc. are produced.
In the corona transfer, a recording sheet is applied with a corona charge of the opposite polarity to that of the toner particles in the toner image. The charge applied to the recording sheet overcomes the attraction of the latent image to the toner particles and electrostatically pulls them onto the recording sheet. Therefore, the transferring process is considerably affected by an ambient moisture which changes the electrical resistance of the recording sheet. Also, when the resistance of the recording sheet is low, the transfer efficiency of the toner images to the recording sheet is undesirably reduced.
The corona discharge utilizes only 5-30% of a supplied electric current for charging the photoconductive surface and a recording sheet, and the greater part of the supplied current is lost through a shield plate. Thus, a charging means and a transferring means utilizing corona discharge are low in the power efficiency. Therefore, a large quantity of power and a high-voltage transformer with high capacity are required for obtaining a desired effect.
To eliminate the above problems in corona discharge, an image forming method employing a brush charging means and a roll transferring means have been proposed.
Further, a requirement to develop small-sized imaging machines has been recently increased. To meet the increasing requirement, it is important to minimize the developing parts. As a proposal realizing the minimization, a developing roll with no sleeve has been proposed to attractively retain magnetic developer on the permanent magnet surface directly and transport the retained magnetic developer to the developing zone by the rotation of the permanent magnet only (JP-A-62-201463).
FIG. 1 is a schematic view showing a sleeve-less image forming apparatus employing a brush charging means. In FIG. 1, a magnetic developer 2 mainly comprising a toner and a magnetic carrier is stored in a developer storage 1. A cylindrical permanent magnet member 4 is rotatably disposed in the lower portion of the developer storage 1. The permanent magnet member 4 has on its exterior circumferential surface a plurality of magnetic poles extending along the axial direction, and at least the surface thereof is made electrically conductive.
The permanent magnet member 4 is formed from a resin-bonded magnet comprising a ferromagnetic powder and a resin as disclosed in JP-A-57-130407, JP-A-59-905, JP-A-59-226367, etc. The surface of the permanent magnet member 4 is made electrically conductive by coating or plating a conductive layer on the surface, or by adding an electrically conductive material during kneading the starting material. A semi-conductive permanent magnet member made of a hard ferrite magnet may be also used.
A photoconductive drum 3 which is rotatable in the direction indicated by an arrow is disposed opposite to the permanent magnet member 4 with a developing gap (g). The thickness of the magnetic developer layer magnetically attracted on the surface of the permanent magnet member 4 is regulated by a doctor blade 5 which is attached to an end portion of the developer storage 1 with a doctor gap (t). A brush charging means 6, a transfer roll 7, a cleaning means 8 and a blade 9 are disposed around the photoconductive drum 3. The magnetic developer 2 attracted on the permanent magnet member 4 is biased with direct current from an electric source (not shown) through the permanent magnet member 4 or the doctor blade 5.
Upon rotating each of the photoconductive drum 3, permanent magnet member 4 and transfer roll 7 in the direction indicated by an arrow, the surface of the photoconductive drum 3 is brushed with the charging brush of the brush charging means 6 to be charged to a uniform potential. The charged portion of the photoconductive drum 3 is exposed to a light image (not shown) to record an electrostatic latent image corresponding to the original information to be reproduced. Separately, the magnetic developer 2 is attracted on the permanent magnet member 4 and transported to a developing zone defined by the space between the photoconductive drum 3 and the permanent magnet member 4. In the developing zone, a toner in the magnetic developer 2 is deposited on the electrostatic latent image by the electrostatic attraction of the latent image to form a visual toner image on the photoconductive drum 3.
The visual toner image is transferred to a recording sheet 10 by the transfer roll 7. The transferred toner image moves in the direction indicated by an arrow and is fixed to the recording sheet 10 by a fixing means (not shown). The toner remaining on the photoconductive drum 3 after transferring step is recovered into the cleaning means 8 by the blade 9 contacting with the surface of the photoconductive drum 3.
However, In the above conventional image forming method, the magnetic carrier in the magnetic developer 2 is likely to adhere to the photoconductive drum 3. The adhered carrier passed through the blade 9 causes various drawbacks when reaches the charging brush 6. For example, since the magnetic carrier is usually electrically conductive, a leak of charge occurs when the magnetic carrier on the photoconductive drum 3 is brought into contact with the charging brush 6. This causes non-uniform charging of the photoconductive surface, generation of loud noise, image defects such as black spots, etc. and, in the extreme case, involves a danger of fires.
A solution for the above problems may be to tightly and strongly press the blade 9 on to the surface of the photoconductive drum 3 to completely remove the adhered carrier. However, this is likely to damage the photoconductive surface and decreases the life of the photoconductive drum 3. Such problems caused by the adhered carrier on the photoconductive drum 3 becomes more serious in a small-sized image forming apparatus omitting a cleaning means 8.